This invention relates to a pneumatic tire, and more particularly, to a pneumatic radial tire of which the ply steer which is generally high in radial tires is reduced and which has improved straight driving characteristics.
Conventional radial car tires generally have a construction in which a belt reinforcing layer consisting of at least two layers is disposed between the tread and the carcass cord layer substantially parallel to the circumferential direction of the tire. The reinforcing cord of one of the layers of the belt reinforcing layers has an angle of 15.degree.-30.degree. with respect to the tire circumferential direction and the other makes an angle of 150.degree.-165.degree. with respect to the tire circumferential direction, these reinforcing cords cross each other, and the carcass cord layer consists of a single or double layer with the cord of each layer being substantially at right angles to the tire circumferential direction. The radial tires of this kind have better braking performance and wear resistance and a lower fuel consumption due to the effects of the belt reinforcing layers, but have a shortcoming in that their straight driving performance is inferior due to the belt reinforcing layers. In other words, there is a lateral force to the right or left with respect to the driving direction when the radial tire rolls forward, even if the slip angle is zero. This lateral force leads the car in a direction different from the direction intended by the driver.
Generally, the lateral force at zero slip angle consists of components of force that are generated by two different mechanisms, one being referrred to as "conicity" (CT) and the other, "ply steer" (PS). They are classified as parts of the uniformity characteristics of the tire. In accordance with the uniformity test method (JASO C607) of car tires, the conicity (CT) and ply steer (PS) can be expressed by the following equations from their definition in conjunction with LFD, LFDw and LFDs, wherein LFD represents the mean value of the lateral force when the tire rotates once, LFDw is the value measured on the outer side and LFDs is the value measured when the tire is turned inside out. EQU LFDw=PS+CT (1) EQU LFDs=PS-CT (2)
From the above equations (1) and (2), PS and CT can be given as follows: EQU CT=(LFDw-LFDs)/2 (3) EQU PS=(LFDw+LFDs)/2 (4)
The relation of these equations (1) through (4) can be expressed as shown in FIG. 1.
The conicity is believed to result from the tire shape being geometrically asymmetric with respect to the center in the circumferential direction of the tire, or is believed to be the force that is generated when the tire, which is in the form like a circular truncated cone, rolls. Since the conicity is affected primarily by the positions of the belt reinforcing layers inserted into the tread of the tire, it can be reduced by any improvements in or relating to the tire production. By contrast, the ply steer is a peculiar force resulting from the structure of the belt reinforcing layers and hence, it has been believed substantially difficult to markedly reduce the ply steer unless the structure of the belt reinforcing layers themselves are changed.
Now, the belt reinforcing layer will be discussed in particular. The belt reinforcing layer can be expressed as a two-layered laminated sheet 50 consisting of belt reinforcing layers 50u and 50d, such as depicted in FIG. 2(A). It is well known that when a tensile force is permitted to act upon this two-layered laminate sheet 50 in the tire circumferential direction EE', the two-layered laminate sheet 50 undergoes deformation not only inside the two-dimensional plane in which the tensile force acts but also three-dimensionally outside the plane, and finally undergoes torsional deformation as depicted in FIG. 2(B). The ply steer results from this torsional deformation of the belt reinforcing layer.
Various attempts have been made in the past to reduce the ply steer by adding a new belt reinforcing layer to the existing belt reinforcing layers, but the addition of a new belt reinforcing layer deteriorates performances of the radial tire such as the low fuel consumption characteristic, increases the tire weight, lowers the efficiency and raises the cost of the tire production; and it is not preferable.